The invention relates to a method of making a negative exposure mask in which a pattern of masking material is formed on a transparent substrate.
Masks of this type are used in the lithographic transfer of patterns in semiconductor technology. The lithographic transfer of patterns has been developed because the integrated circuits require patterns with extremely small dimensions and tolerances. All types of the lithographic pattern transfer need a radiation source and a mask for modulating the radiation in such a manner that the respective pattern is transferred to a substrate which is coated with a resist material that is sensitive with respect to the radiation.
In view of the precision and fine details which are needed for generating this photoresist pattern in semiconductor technology, highest demands are made on the manufacture of the process masks involved, particularly with respect to high image fidelity. Formerly, the process masks were made substantially in accordance with a subtractive or an additive method. In the substractive method (direct mask production) a positive-working photoresist layer was applied onto a glass plate coated with an approx. 500-1000 A thick metal layer, which positive-working photoresist layer was then exposed through an original with negative mask motifs, e.g., via an optical system or by maintaining a defined distance between original and photoresist layer, and subsequently developed. The metal not covered by the photoresist was etched off, and, after the removal of the photoresist, the thus made mask was applied to its use.
In the additive process (lift-off method) the metal was vapor-deposited after the photoresist process. In detail, a cleaned glass plate was coated with a positive-working photoresist, and using an original with positive mask motifs, the resist layer was exposed and subsequently developed. Then, metal was vapor-deposited on the glass plate and the photoresist islands, and the photoresist islands with the metal layer covering them were removed in a lift-off process. After additional cleaning the mask was applied to its use.
The disadvantages are that in both processes, the substractive as well as the additive one, the photoresist layer is exposed in accordance with the shadow masking process, with a defined distance between mask and photoresist layer being maintained. In this kind of exposure there can be a diffraction at the mask, and the resulting "exposure sub-structure" is highly undesirable when patterns of smaller and smaller geometry and greater fineness of details are to be imaged. Another disadvantage of the subtractive process described above is that, for making rectangular geometries, originals with negative rectangular mask motifs are less suited than originals with positive rectangular mask motifs, because for diffraction-geometrical reasons, behind negative and positive mask motifs of equal size there is different photoresist structure, and for the respective photoresist structure, with equal size, the geometry of the positive mask motif can be made larger (i.e., with the diffraction effect being smaller) than when a negative mask motif is used. Negative rectangles made in accordance with the subtractive process show for these reasons strongly rounded-off edges which with smaller geometries and decreasing ratio between the height of the mask material and the width of the narrowest mask motif (the aspect ratio) can even result in a circular shape.
The above specified disadvantages are avoided in the additive process where originals with positive mask motifs are employed, but thin photoresist layers have to be used to achieve a high image fidelity. The use of thin resist layers has the disadvantage that the photoresist islands produced therefrom and the metal layer coating, are quite difficult to lift off from the substrate.